The present invention relates to a method and apparatus for measuring the iron content in layers of zinc and/or the thickness of the layer of zinc in the case of galvanized steel by the use of a source of X-rays and a radiation detector. In the production of steel, strip steel is coated with zinc in order to increase its corrosion resistance. This coating is effected, for instance, in a so-called galvannealing process, i.e. by subsequently heating the galvanized steel to about 500.degree. to 600.degree. C. As a result of this "galvannealing," iron diffuses into the layer of zinc so that intermetallic bonds between zinc and iron are formed. This intermetallic bond is of extremely great importance for the further workability of the coated steel and the adherence of the layer of zinc. In this connection, it is particularly important to adjust the proportion of iron in the zinc layer within narrow limits during the manufacturing process, for which purpose the precise percentages of the amount of iron in the layer of zinc must be known in each case. In order to determine this percentage of iron or to measure the iron content in the zinc layer, a method of measuring the crystallographic lattice constants for the Fe/Zn crystallization by the Bragg method has been described by Kawasak Seitetsu Giho under the title "Continuous Measurement of Fe Content in Galvannealed Coating," dated Jan. 28, 1988. However, this method is suitable only for the laboratory, since the expense for apparatus is very high, the measurement times are relatively long, namely more than 20 seconds, and a change in position of the material being measured leads to erroneous measurement results.
A method is also known in which a material radiation excitation is produced by an X-ray radiator so that an energy-selective measurement of the percentages of zinc and iron can be effected by means of a proportional counter. This method has been described by Schikawa Works Nisshin Steel Co. Ltd., Japan, under the title "Measuring the Degree of Alloying of Galvannealed Steel Sheets by X-ray Diffraction Technique and Its Practical Use." By this method, the proportional counter (detector) is brought during the measurement process at different angles to the surface of the material being measured so that the crystal structure can be determined from these measurement results. In this connection, very considerable problems as to stability arise so that, for instance, the mere movement of the material being measured, i.e. the movements of the material around the resting surface, necessarily leads to erroneous measurements. Thus, this method also is merely suited for the laboratory.
None of the methods known up to now for measuring the content of iron in zinc layers in galvanized steel is suitable for use for measurements during the production process in order thus to note the actual value, so that the process control can be carried out accordingly.